Chloride concentration control in immersion copper coating

ABSTRACT

The bath life of an aqueous acidic copper coating solution is increased by maintaining chloride ion concentration within critical limits during operation of the coating process.

[ Mar. 14, 1972 United States Patent Reed et a1.

References Cited UNITED STATES PATENTS [72] Inventors: Stuart Reed, Warminster; Louis Schiffman, Abington, both of Pa.

2,217,921 Saukaitis............................117/130 R 7 AU A h Prod ts,1 .,A bl ,P.

l 3] wgnee mc em m er a Primary Exammer-Ralph S. Kendall Filed: Oct. 8, 1970 Attorney-Emest G. Szoke [21] Appl.No.: 79,101

[57] ABSTRACT The bath life of an aqueous acidic copper coating solution is [52] US. CL.......,.......r.,.....,...m...,.l17/130 E, 72/47,106/1,

increased by maintaining chloride ion concentration within critical limits during operation of the coating process.

1 Claims, 4 Drawing Figures [58] Fieid of Search COATING WEIGHT MILLTGRAMS PER 80. FT.

CHLORIDE CONCENTRATION --|5.| GRAM-IONS/LITER 500 --9 GRAM-IONS/LITER -s GRAMIONS/ LITER IO I5 3O 4O 5O 6O FERROUS IRON CONCENTRATION GRAM-IONS PER LITER FIG. I

INVENTOR ATTORN PATENTEDMARM I972 3 649,343

SHEET UF 4 25 5 BATH A r F- /BATH a 4m {E520 5 35 80. I5

2 9S i E 9 $25 5 5 WEIGHT OF STEEL COATED IN POUNDS (THOUSAND) FIG. 4

ATTOR CHLORIDE CONCENTRATION CONTROL IN IMMERSION COPPER COATING BRIEF DESCRIPTION OF THE INVENTION This invention relates to the deposition of copper from an aqueous acidic solution ofa copper salt, onto the surface ofa metal with a higher electrode potential than that of copper. More particularly, the invention concerns increasing the life of the coating bath by maintaining chloride ion concentration within predetermined bounds. This requires that the concentrate for makeup of the original bath as well as the composition employed to replenish the bath contain certain initial and predetermined concentrations of chloride so that chloride ion concentration may be established and controlled in the bath within predetermined critical limits.

BACKGROUND OF THE INVENTION In the art of ferrous metal working, the metals are rolled, drawn or otherwise subjected to pressures to change their size and shape. In subjecting the metal to such pressures a lubricating film or coating is first deposited on the surface ofthe metal so that the effect of frictional forces which develop may be avoided. A relatively ductile and soft metal such as copper can provide suitable lubrication when deposited on the metal by plating the copper out onto the metal surface by immersion deposition from a suitable aqueous acidic solution ofa copper salt.

The preparation of copper coating baths and the substantially dry admixtures which are dissolved in water or in acid solutions to prepare or replenish the baths are well known in the art. Baths of this type and their manner of use are disclosed, for example, in U.S. Pat. No. 2,217,921 which is incorporated herein by reference. The makeup formulation, as a dry powder usually contains a copper salt, an inhibitor to prevent extensive acid attack, halogen ions in halogen salts and soluble fillers. The makeup formulation is added to an aqueous acidulated solution consisting of water and a small amount of a mineral acid. The mineral acid is present to activate the ferrous metal surface.

As the coating process progresses, the copper content of the bath diminishes through its inclusion in the coating deposited on the surface. The specific losses of the coating producing ingredients must be replaced so that the coating bath is maintained within its prescribed operating limits. An operating bath is usually replenished with the original makeup formulation and the amount used for replenishing depends on how much copper has been depleted. Since there are other ingredients in the makeup formulation, they too are carried into the bath when replenishment occurs. The bath is eventually dumped when coating color or quality deteriorates or coating weight drops off to an unacceptable level. When an operating bath is dumped it usually still contains quantities of essential ingredients sufficient to produce adherent coatings which have some lubricating ability.

It is present day practice to determine when replenishment should occur and when a coating bath should be dumped, by measuring the amount of ferrous ion in the bath. Iron is supplanted on the surface ofthe metal by an equivalent amount of copper, on a mole basis. so that the ferrous ion concentration increases as the bath is worked. The increase in iron concentration can be used as a measurement of how much copper should be added to bring copper content up to the required level. The bath is dumped when ferrous ion concentration reaches a certain level, for from experience the coating quality produced thereafter becomes unacceptable.

U.S. Pat. No. 2,217,921 teaches that the coating bath is insensitive to wide variations in halide ion content, except that too low-halide concentration in the bath results in nonadherent coatings and too high a concentration slows down the coating process considerably.

We have discovered that the chloride concentration in the coating bath. far from being unimportant, is actually the single most important parameter for increased coating bath life. By

critical control of chloride ion concentration, a bath may be replenished many more times than is normally practiced, without loss of coating quality. As a result, a coating bath can be operated for a longer period of time at the same rate of throughput.

The principal object ofthis invention is to provide a coating process which will prevent early and wasteful dumping of copper-coating baths.

An added object of this invention is to increase the work load in terms of total surface area coated by the same copper coating bath.

Yet another object of this invention is to provide a coating process which produces good quality copper coatings using less copper in the makeup material and to produce good visible quality coatings with lower coating weights. Other and further objects will be apparent from the detailed description of the invention which follows.

DETAILED DESCRIPTION This invention increases coating bath life of an aqueous acidic coating solution of a copper salt by maintaining chloride ion concentration within certain predetermined critical limits. The chloride concentration, when controlled in accordance with the teachings of this invention, allows for the production of coatings essentially uniform in weight throughout the life of the bath. The bath life is extended considerably which ameliorates the considerable waste disposal problem inherent in the present day operation of copper coating baths.

During the working of a bath, the ferrous ion concentration in the bath is measured at certain intervals. This measurement is made in order to determine when to replenish the bath with a dry admixture ofthe original makeup formulation. Iron content is readily determined by a titration ofa bath sample with 0.1 N potassium permanganate. A 1 ml. titration corresponds to 5.6 grams/liter concentration of ferrous ion.

This invention requires that the original makeup and replenishment material contain critical amounts of chloride. In this way, chloride can be added at each replenishment step at a predetermined rate as a function of iron buildup in the bath. In particular, chloride ion can be added at an overall rate of from about 0.5 gram-ion/liter to about 6 gram-ions/liter for each 5.6 gram-ions/liter increase in ferrous ion concentration. Preferably, chloride ion should be added at an overall rate of from about 0.8 to about 2.0 gram-ions/liter for every 1 ml. increase in iron titration or 5.6 gram-ions/liter increase in ferrous ion concentration. This addition may be continued until total chloride concentration reaches from about 35 gramions/liter to about 40 gram-ions/liter at which time the bath is no longer useful.

The chloride salt in the makeup material should be that amount which, when added to water or the aqueous acidulated solution, will produce a chloride concentration in the coating solution of not more than 6 gram-ions/liter. Preferably, the initial chloride concentration should be from about 2.5 to 4.5 gram-ions/liter for acceptable coating weights to be initially produced and to facilitate a prolonged coating bath life.

A makeup material containing a source of copper, an inhibitor, and a water-soluble chloride salt is to be used for the bath startup and can be used as well for replenishing the bath as necessary. A replenishing material consisting essentially of the same constituents, but present in different proportions, can be employed to replenish the working bath.

During the coating operation, the depleted copper in the coating solution must be replaced to maintain the bath within its optimum operating limits. The bath is maintained by suitable addition of makeup concentrate or replenishing concentrate. For proper replenishment to be accomplished, chloride and copper should be added in definite proportions. As we have indicated, for each I ml. increase in iron titration or 5.6 gram-ions/liter increase in ferrous ion concentration, from about 4 g./l to about g./l of copper should be added. For each part by weight of copper added. from about 0.1 to about 0.6 parts by weight of chloride should be added to the operating bath. In the makeup material and replenishment material the amounts of copper and chloride present should be within these definite weight proportions. i.e., for each part by weight of copper present. from about 0.1 to about 0.6 parts by weight ofchloride should be present.

The source for copper can be any conveniently available copper salt employed in the art. Preferred sources for the ad dition of copper to the coating bath are copper sulfate monohydrate, copper sulfate pentahydrate. tetraamine copper sulfate, or copper ammonium sulfate heptahydrate.

The amount of copper used in the bath may vary over a fairly wide range depending, of course, on quality and coating weight desired. As results cited herein demonstrate, one of the advantages of the present invention is that it is possible to get high-quality coatings, previously requiring high-copper concentrations in the bath, by using much lower quantities of copper. For example, an original makeup bath containing as little as 10 lbs. of copper sulfate monohydrate per 100 gallons of bath or lbs. copper ammonium sulfate heptahydrate per lOO gallons of bath will produce coatings of high quality in the preferred weight range of from about 650 mg./ft to about 1.400 mgJft? The source for chloride ion in the aqueous acidic coating solution can be any conveniently available water soluble chloride salt. Examples ofchloride salts which can be used are sodium chloride. potassium chloride and ammonium chloride.

The inhibitors employed in the process of the present invention are those which regulate the deposition of copper on the metal surface and control the acid attack on ferriferous surfaces. Basic organic nitrogenous substances have proven to be most effective as inhibitors for copper coating processes.

We have found the Schiff Bases to be highly effective as inhibitors. particularly the reaction product of aliphatic aldehydes and aromatic amines. An example of an inhibitor l have employed with good results is the condensation product of orthotoluidinc and formaldehyde.

The amount of the particular inhibitor in the concentrated formulations specified herein can vary from 0.5 to 3 percent by weight depending upon the strength of the inhibitor that is employed.

The inhibitor present in the coating will be depleted during operation of the process by such factors as dragout and Chemical breakdown. it is common practice to include the inhibitor in the replenishing admixture to replace the inhibitor loss.

The improved process of this invention is normally em ployed after cleaning of the metal surface has been accom plished. The cleaning steps can be carried out by conventional methods which form no part of the present invention. A conventional alkaline cleaner can be employed followed by a water rinse. Should the surface be heavily soiled, a detergent cleaner additive can be employed in the cleaning step. When scale or rust is present on the surface, a conventional pickling operation would be necessary followed by a water rinse.

in the coating operation. the substrate is brought into contact with the aqueous acidic coating solution under suitable conditions of pH. temperature. and contact time.

The time of treatment of the metal surface with the coating solution need only be long enough to insure complete wetting of the surface and can be as long as 5 minutes. Preferably, contact time between the surface and the coating solution should range from about 1 minute to 5 minutes.

The coating solution is preferably applied by conventional immersion methods. ln certain specialized applications, spray or flow-coating techniques can be employed. The coating solution should be maintained at a pH below 2.0.

The strong mineral acid employed to make up the initial aqueous acidulated solution should be selected from the group of sulfuric, phosphoric and nitric acid. Hydrochloric acid can be employed as the strong mineral acid to make up the initial aqueous acidulated solution. Should hydrochloric acid be employed as the source of strong acid, the chloride ion present through addition of the acid must be taken into account when the makeup admixture is added to the aqueous acidulated solution. The use of hydrochloric acid is not preferred since it complicates the critical control of chloride ion concentration during the coating process.

The temperature at which the coating process can be operated is from about to about F. It is preferable to operate the coating bath at a temperature of from about l00 F. to about l50 F.

The normal levels of cations found in the usual commercial water supplies do not have to be eliminated from the coating solution prior to use since these ions have little effect on the coating process. Likewise, anions such as sulfate do not affect the coating process and, in fact, sodium sulfate is a commonly employed soluble filler in the concentrated admixtures disclosed herein.

The treating solution can be prepared using a concentrated liquid admixture or a dry powder makeup formulation which, when added to an aqueous acidulated solution, will produce the operative bath with constituent concentrations within the preferred ranges.

The following examples of concentrated admixtures suitable for dilution with water or an aqueous solution to make a coating solution or optimum quality as well as to replenish an operating coating bath are presented:

The coating solution can be replenished with a concentrated admixture having constituents present in different proportions than the makeup material. Formula 4 is presented as an example of such a concentrated admixture suitable for replenishing an operating coating bath.

FORMULA 4 71 by weight (upper sulfate monohydrute 26.5 Copper sulfate pentahydraic 55.9 NaCl so Inhibitor 2.47 Sodlum sulfate 9 l] The importance of precise control of chloride ion, in both the original bath as well as in the replenishment material added to the bath during the course of operation, can best be understood with reference to the following operating procedure. results and accompanying plot.

EXAMPLE I The purpose of this work was to produced by a bath as a function of iron ion concentration in centrations. The results are illustrated in FIG. I.

2 liter baths were prepared with a makeup formulation consisting of 56.1 percent by weight of CuSO monohydrate, 2 percent by weight of inhibitor, 18 percent amount offerrous ion already in the bath.

Low carbon hot rolled 1% X 3 inches steel strips were coated. The strips had been pickled in inhibited H SO just long enough to remove scale.

FIG. 1 illustrates the effect and importance of chloride concentration on coating weight. It demonstrates that at a given ferrous ion concentration, coating weight will differ for each bath with different chloride concentrations. The coating baths are, therefore, not insensitive to wide variations of halide concentrations.

With reference to FIG. I, it can be seen that constant coating weight at some preferred value, i.e., 1,000 mg./ft will run through a number of single points on the chloride concentration curves. Thus, by appropriately increasing chloride content at a carefully selected rate, coating weight can be maintained at a relatively constant value despite an increasing iron invention to coating operations is ilexamples.

EXAMPLE II Copper coating baths were prepared using formulations A and B respectively, each with different concentrations of copper. inhibitor and chloride. Replenishment was made with formulations identical to the respective makeup formulation.

FORMULA A 9? by weight Copper sulfate monohydrate 660 NaCl 30.6 Inhi itor 3.4 (Reaction product oforthotoluidine formaldehyde) FORMULA B 7: by weight Copper sulfate monohydrate 56.1 NaCl 18.0 Sodium sulfate anhydrous 23.9 Inhibitor 20 (Reaction formaldehyde) Formula A is a commonly used copper coating formulation. Formula B is a copper-coating formulation where chloride product of orthotoluidine F. was maintained The panels were point at which the coating'produced by the-bath was nolonger acceptable requiring that the bath be dumped.

TABLE 2 Ferrous ion Coating Chloride ion Appearance of Concentration Wt. Concentration copper' g.-.ions/l. mgJft. g.-ions/l. Coating O-Fresh Bath 934 4.4 Bright 8; Copper 6.72

1.059 8.8 11.76 1.104 13.1 16.80 1,120 17.5 21.28 1.045 21.8 25.20 1,008 26.2 29.12 946 30.6 33.04 894 35.0 36.40 883 39.4 42.00 834 43.8 Dark Red-brown and Dull The results ofa coating operation when Formula B was employed are illustrated in FIG. 3 and Table 3. FIG. 3 plots coating weight produced during the coating operation as a func- Table 3 illustrates the quality and appearance ofthe coating produced during the coating operation.

TABLE 3 Ferrous ion Coating Chloride ion Appearance of Concentration Wt. Concentration Copper g.-ions/l. mg./ft.-' g.-ions/l. Coating (1-Fresh Bath 801) 2.6 7.28

Bright & Cuppery 806 5.2

employed in the coating 17.92 832 10.4 2|.li4 846 13 25.76 B42 7 18.l6 645 39.1 49.84 664 41.7 Dark Red-hrown and Dull lt can be observed from FIGS. 2 and 3 that when Formula B was employed in the coating operation, which allows for continuous control of chloride addition to the bath, a more uniform coating weight was maintained as the coating operation progressed.

As Tables 2 and 3 indicate, a good quality coating was maintained when Formula B was employed, when iron content had far exceeded 42 gram-ions/liter. With much lower, but more uniform coating weights, the production of high-quality copper coatings was possible for longer periods of time when Formula B was used.

The preceding results illustrate the importance of chloride control during a coating bath operation. The makeup formulation. in this case Formula B, must contain a specific quantity of chloride so that when used to prepare a bath and for replenishment. the rate at which chloride concentration will increase in the bath can be predetermined and controlled.

EXAMPLE lll Coating baths A and B were employed to demonstrate the longer bath life capability, in a practical application, when increasing chloride concentration is critically monitored. It can be observed from the following results that substantially increased bath life is obtained while equal or better quality coatings are maintained.

A 1,200 gal. coating bath (Bath A) was prepared with Formula A employed as the makeu material and replenishment material. 335 lbs. of Formula A were added to a bath at 2.5 percent B.V. H 50, and was operated at l20 F. Coating time was from about 2% min. to about 3 min. It was necessary to dump the bath when iron titration with O.l N potassium permanganate reached 4.8 ml., and ferrous ion concentration was 26.88 gram-ions/liter.

Another L200 gal. coating bath (Bath B) was prepared, with Formula B employed as the makeup and replenishment material. 250 lbs. of Formula B were added to a bath at 2.5 percent BM. H SO and was operated at 120 F. with a coating time from about 2 /2 min. to about 3 min. It was necessary to dump the bath when iron titration with 0.1 N potassium permanganate reached 7.3 ml, and ferrous ion concentration was 40.88 gram-ions/liter.

(Reaction product of orthotoluidinc H6. 4 plots chloride concentration as a function of the weight of steel coated during the operation of the respective baths.

The data illustrates the improved results obtained when Formula B was employed where chloride concentration was increased at a predetermined rate within critical limits.

The chloride control during the operation of Bath B enabled operation of the bath to continue for more than double the time, and the weight of metal coated was more than twice that coated by coating Bath A.

We claim:

1. in a process for the electroless deposition of copper from an aqueous acidic coating solution ofa copper salt onto ferrous metal surfaces to provide suitable lubrication to the metal surface which will be subsequently subjected to pressures to change its size and shape, wherein the metal surface is immersed for a period of time no longer than 5 minutes in the coating solution at a pH below 2.0 and at a temperature from about F. to about F., said coating solution consisting essentially of a copper salt selected from the group of copper sulfate monohydrate, copper sulfate pentahydrate, and copper ammonium sulfate heptahydrate, a basic organic nitrogenous inhibitor, a water-soluble chloride salt, and a strong mineral acid selected from the group of sulfuric, phosphoric and nitric acid, and wherein as the coating process continues iron is supplanted on the surface ofthe metal by an equivalent amount of copper thereby causing a steady increase in ferrous ion concentration in the acidic aqueous solution; the improvement which comprises increasing the life ofthe acidic aqueous solution by using as the makeup material for said coating solution a dry makeup admixture consisting essentially ofa copper salt, a basic organic nitrogenous inhibitor and a water soluble chloride salt which when added to an aqueous acidulated solution consisting essentially of water and a strong mineral acid selected from the group of nitric. sulfuric and phosphoric acid provides an aqueous acidic coating solution such that for each part by weight of copper in said coating solution there is present from about 0.1 to about b 0.6 parts by weight of chloride and the chloride ion concentration initially does not exceed 6 gram-ions/liter, and thereafter replenishing the said aqueous acidic coating solution by the periodic addition of sufficient amounts of a dry replenishing admixture consisting essentially of a copper salt, a basic organic nitrogenous inhibitor, and a water-soluble chloride salt, at a rate such that the chloride ion concentration in the aqueous acidic coating solution is increased by the addition of the dry replenishing admixture from 0.5 gram-ions/liter to 6 gram-ions/liter for each 5.6 gram-ions/liter increase in ferrous ion concentration. 

